AtT-20 mouse pituitary tumor cells synthesize and secrete corticotropin, beta-lipotropin, and several peptides with opiate activity, beta-, alpha-, and gamma-endorphins. Aspects of the regulation of biosynthesis and secretion of these peptide hormones are being studied in this clonal cell line, which is a useful model system for the corticotropin-beta-lipotropin producing cells of the anterior pituitary gland. Studies during the past year have concentrated on the secretion of beta-lipotropin and beta-endorphin by the cells, including (1) the packaging of these peptides into secretory granules, studied by immunocytochemistry, (2) the inhibition of the secretion of peptide hormones by glucocorticoids, acting through glucocorticoid receptors, and (3) studies on the electrical excitability of AtT-20 cells and its enhancement by dibutryryl cyclic AMP.